Number of documents handled in offices is growing, and these growing numbers of documents should be managed efficiently. In order to efficiently manage these documents, the need of the electronic filing system, which converts paper documents to electronic image data using an image scanner, is growing.
Color documents handled in offices are also increasing. In accordance with such a situation, the image scanners used for electronic filing are also required to cope with color processing.
When color image data electronified by a color image scanner is used in an actual case, the color image data is output to any of different types of output devices, for example, a video monitor such as CRT, a color printer, or a printing machine. Under such situations, there is some problem such that output colors between different devices do not match even if the same data is used to output images from these devices. That is, there is a different color tone between a monitor display and a color print.
The difference between colors output from output devices is caused by a difference between color spaces expressed by the input/output devices including the scanner, because each input/output device has specific characteristics to express its own color space.
In order to solve the above-mentioned problem, a color management system using ICC profile has been introduced in recent years. This ICC profile is proposed by International Color Consortium (ICC). The ICC profile is so called “history” in which color space and characteristics of each input/output device are written. The color management system on a personal computer determines the characteristics of each input/output device using the ICC profile and performs transaction of color data with the input/output device. This color data has been subjected to color correction so that colors between two input/output devices are apparently the same for the human eye.
The image scanner as an input device uses the ICC profile specific to the scanner. Therefore, in the color management system using the ICC profile, a user is required to manage relative information among the stored color image data, the scanner used to scan the color image data, and the ICC profile used for the scanning, and to provide corresponding color image data when the data is reproduced. If the management is troublesome for the user, it is possible to embed the ICC profile in a header of color image data to be stored. In this case, however, the size of the image data is increased and there by efficiency of electronic filing is decreased.
The International Electrotechnical Commission proposed “sRGB” as a default red, green, blue (RGB) space that was standard for the Internet at IEC/WD6 1996-2-1. The sRGB is described in “Default RGB color space—sRGB” as color management in a multimedia system. This sRGB is a standard color space that is device-independent.
Since the sRGB is the device-independent color space, by converting image data output from a color image scanner to the sRGB, the color management system can handle the image data specific to the color image scanner as the sRGB that is the standard color space. Further, the user does not need to manage the relative information among the stored color image data, the scanner used for scanning the color image data, and the ICC profile used for the scanning. Thus, the operation becomes extremely efficient.
Color image scanners provided with a function of outputting an SRGB image as the standard color space have been increasing in recent years for the above reason. Most of such scanners have a color conversion function in order to output sRGB images. This function is used to convert scanned device-dependent RGB signals to device-independent sRGB signals.
As the color image scanner is been used, a document-illuminating lamp, for example, is being degraded over time and thereby color tone of output images is being changed. In such a case, calibration is generally performed to stabilize the color against a color change. Calibration is to always keep color constant by controlling a driver of the illumination lamp to correct light quantity by the quantity that has been reduced, or by adjusting parameters of the color conversion function.
The calibration of the color conversion function is implemented at an appropriate time when the color tone has changed and it is determined that this change is beyond an allowable level. However, it is difficult to accurately determine by the scanner whether the changed color is beyond the allowable level within a short time as a case where the human can do. In other words, the human eyes can sense even a slight difference between colors and determine the difference very quickly and accurately. It has been general to use a method of totalizing operating time of the scanner and illuminating time of the illumination lamp, and of implementing calibration when the operating time of the color scanner reaches a time more than a predetermined time based on the totalized time. Therefore, according to such a method, a timing at which the calibration should be implemented may be missed, processing has been executed while the color tone has been changed, and degraded data has sometimes been continuously output.
Conventionally, even the color image scanner provided with the function of outputting sRGB images has used such an ordinary method based on the operating time of the scanner. Therefore, it has been desired to provide a method of executing calibration of any devices, which output sRGB images, at more accurate timing.